Why do we exist?
Abandoning the fundamental distinction between matter and
antimatter means that the two states can convert to each
other. It may also solve one of the biggest mysteries of our universe:
where has all the antimatter gone? After the Big Bang,
the universe was filled with equal amounts of matter and antimatter,
which annihilated as the universe cooled. However,
roughly one in every 10 billion particles of matter survived
and went on to create stars, galaxies and life on Earth. What
created this tiny excess of matter over antimatter so that we
With Majorana neutrinos it is possible to explain what
caused the excess matter. The hot Big Bang produced heavy
right-handed neutrinos that eventually decayed into their
lighter left-handed counterparts. As the universe cooled, there
was insufficient energy to produce further massive neutrinos.
Being an antiparticle in its own right, these Majorana neutrinos
decayed into left-handed neutrinos or right-handed
antineutrinos together with Higgs bosons, which underwent
further decays into heavy quarks. Even slight differences in the
probabilities of the decays into matter and antimatter would
have left the universe with an excess of matter.
It is encouraging that we have seen
such a phenomenon recently. In the
past three years, the KTeV experiment
at Fermilab near Chicago and the
NA48 experiment at CERN have established
that the neutral kaon a
bound state of a down quark and antistrange
quark and its antiparticle
decay in a slightly different manner. At
only one part in a million, this difference
is very small. However, we only
need one part in 10 billion for us to
exist. If a similar difference in the decay
probabilities exist in right-handed neutrinos,
which is quite likely, it could have
produced a small excess of primordial
matter from which all the other particles
have been formed.
Neutrinos are everywhere. Trillions of them are passing through
your body every second,but they are so shy and we do not see or feel
them. They are the least understood elementary particle we know
- Birth of Neutrinos
Existing of neutrinos was suggested as a "desperate remedy" to the
apparent paradox that the energy did not appear conserved in the
world of atomic nuclei.
- The Standard Model
The Standard Model of particle physics can describe everything we
know about elementary particles. It says that neutrinos do not have
mass. Neutrinos do not have mass because they are all "left-handed"
and do not bump on the mysterious "Higgs boson" that fills our
- Evidence for neutrino mass
In 1998, a convincing evidence was reported that neutrinos have
mass. The Standard Model has fallen after decades of invicibility.
The evidence comes from experiments deep underground in pitch
darkness with many thousands of tonnes of water housed in mines.
- Implications of neutrino mass
Neutrinos are found to have mass, but the mass is extremely tiny, at
least million times lighter than the lighest elementary particle:
electron. How do we need to change the Standard Model to explain
the neutrino mass? Some argue that our spacetime has unseen spatial
dimensions, and we are stuck on three-dimensional "sheets". Other
argue that we need to abandon the sacred distinction between matter
- Why do we exist?
When Universe started with the "Big Bang", there were almost equal
amount of matter and anti-matter. Most of matter was
annihilated by anti-matter when Universe cooled. We are
leftover of one part in ten billions. Why was there a small excess
matter over anti-matter so that we can exist? Once we abandon the
sacred distinction between matter and anti-matter, it provides a
key to understand why we exist.
The mysteries about neutrinos are now being unraveled dramatically.
We will learn much more in the coming years.
This homepage is based on Feature Article "Origin of Neutrino mass"
in Physics World, May 2002, by Hitoshi
Murayama. The whole article can be download as a PDF file.
Last modified: Fri Jul 5 11:06:49 PDT 2002